Roadway signs, such as highway signs, markers, advertising displays, etc., have long been constructed using retroreflective sheetings. Retroreflective sheeting reflects incident light rays substantially back toward the source as a cone of light. Thus the light emitted by headlights of a motor vehicle toward a sign constructed with such sheeting will be reflected back toward the vehicle so as to be visible to the occupants of same.
In practice, retroreflective sheetings were typically first employed in roadway signs in the background portions of the sign, with the sheeting being cut out around raised indicia, or being selectively covered, e.g., painted, to produce same. Thus an occupant of an approaching vehicle would first detect the sign's background, and upon close approach, the indicia would become legible due to the contrast of brightness and color between the indicia and background. Such a sign is disclosed in U.S. Pat. No. 2,326,634 (Gebhard et al.) which relates to the retroreflective brilliancy of microsphere-based sheeting and the relationship of the refractive index of the microspheres thereto.
Roadway signs can also be constructed from cube-corner retroreflective sheetings such as disclosed in U.S. Pat. No. 3,712,706 (Stamm) which discloses such sheetings and methods for preparing the same.
Recent constructions employ retroreflective sheeting in both the background region and indicia region. Such combinations typically provide increased long range detectability and recognizability, i.e., conspicuity, to the sign. Legibility is typically provided by coloring one region to provide color contrast, typically resulting in a ratio of retroreflective brightness, i.e., contrast ratio, that is substantially constant over the intended observation distances. For instance, white or silver sheeting may be used as the indicia and green colored sheeting as the background, such as is commonly seen along the interstate highway system in the United States. Such signs typically have a substantially constant contrast ratio of about 5:1 to 6:1, i.e., the indicia region is brighter than the background region by the stated ratios.
A problem with some roadway signs is that they may be difficult to read at night because very bright retroreflection by the background tends to wash out or obscure the indicia, rendering same difficult to see. Thus the maximum brightness of the background which may be achieved tends to be limited by the degree to which it reduces the legibility of the sign, and the maximum legibility which may be achieved tends to be reduced by the degree to which contrast of the indicia with the background is reduced.